The present invention generally relates to temperature compensation circuits in pickup tube devices, and more particularly to a temperature compensation circuit in a pickup tube device comprising a bias lighting device, capable of compensating a bias signal current variation due to temperature variation in the bias lighting device.
Generally, a photoconductive pickup tube of the blocking type having characteristics such as high sensitivity, low visual persistence (residual image), compact, and high resolution, is widely used as a pickup tube for a color television camera. However, in this type of a pickup tube, a capacitive residual image is introduced due to the electrostatic capacitance between a transparent conductive membrane and a photoconductive membrane. This electrostatic capacitance is introduced because the electrical charge of the electrostatic capacitance cannot be discharged with respect to the rapid change of the image. Accordingly, in order to reduce the residual image, a bias lighting device is provided at the front surface of the pickup tube, and a bias light is irradiated on the bias lighting device. The time constant is decreased by maintaining the device biased in the operating range.
The above bias lighting device generally has a construction wherein a plurality of light-emitting diodes are held at equally spaced intervals on a ring-shaped holding frame. However, because the light-emitting diodes are semiconductors, the light emitting characteristic varies according to the ambient temperature variation. For example, if the ambient temperature rises, the light emitting quantity of the bias light increases. Hence, the input light at the front surface of the pickup tube increases even when the luminous intensity of the image does not increase. As a result, the picture becomes bright and the color phase changes to produce an irregular color in the picture, by the pickup output thus obtained.
Particularly, in a color television camera system, a gamma compensation circuit is generally used to achieve correct reproduction of the luminance. However, as is well known, the gamma compensation characteristic of the gamma compensation circuit is a level compression characteristic, and the amplification at low levels of the video signal is exceedingly high. Accordingly, when the input light is low, the output light will vary greatly even when the variation in the input light is slight. Thus, the bias light must be compensated so that the bias light does not vary with the temperature variation, or electrically compensate the variation in the bias light caused by temperature variation in a signal system.
As a conventional circuit for carrying out compensation in accordance with the temperature variation (hereinafter referred to as temperature compensation), there was a temperature compensation circuit in which the light emitting quantity of the bias light is detected and compensation is carried out so that the bias light is always constant with respect to the variation in the bias light due to temperature variation in order to maintain the bias signal current of the bias lighting device constant. However, because this conventional temperature compensation circuit uses a constant current source, a large number of circuit elements are required. In addition, the cost of the temperature compensation circuit becomes high, since a high power source voltage is required. Hence, there was a disadvantage in that the above conventional temperature compensation circuit was unsuited for application in a portable type single-tube color television camera wherein there is a demand for low cost production.